The invention relates to an operating method for a discharge lamp having at least one dielectrically impeded electrode in accordance with the preamble of Patent claim 1 or 3. In particular, it concerns a method for igniting a dielectrically impeded discharge in a discharge lamp.
An operating method for a discharge lamp having dielectrically impeded electrodes, and a corresponding circuit arrangement for carrying out this method, are described in German Patent Application DE 197 31 275.6. During operation, periodically recurring voltage pulses which cause a dielectrically impeded discharge in the discharge lamp are applied to at least one of the dielectrically impeded electrodes. Depending on the geometry of the discharge vessel and the arrangement of the lamp electrodes, the peak value or amplitude of these voltage pulses is between approximately 1 kV and approximately 5 kV. The pulse repetition frequency is in the region of approximately 25 kHz to approximately 80 kHz. The voltage pulses are generated with the aid of a flyback converter and a transformer whose secondary windings are connected to the lamp electrodes. During the conducting phase of the flyback converter switching transistor, there is built up in the transformer a magnetic field whose electric energy is dissipated in the form of voltage pulses to the discharge lamp during the blocking phase of the flyback converter switching transistor. The discharge lamp is operated at its nominal operating voltage during the entire operating time, that is to say also during the starting phase. It has emerged that it is possible for there to form during ignition of a dielectrically impeded discharge in the discharge lamp preferred discharge channels which prevent the production of a dielectrically impeded discharge distributed uniformly over the length of the electrodes and cause an inhomogeneous light emission of the discharge lamp along the discharge vessel. Moreover, in the case of discharge lamps having dielectrically impeded electrodes it is not possible to improve the readiness of the discharge lamp to start by preheating the electrodes or with the aid of an electron emitter, in order by means of thermionic emission or by means of field emission to make available sufficient free charge carriers for igniting the discharge.
It is the object of the invention to specify an operating method for a discharge lamp having at least one dielectrically impeded electrode which ensures reliable ignition of the dielectrically impeded discharge in the lamp. In particular, the aim is to prevent the formation in the discharge lamp of individual, preferred discharge channels which would cause inhomogeneous light emission of the lamp.
This object is achieved according to the invention by the features of Patent claim 1 or 3. Particularly advantageous designs of the invention are described in the subclaims.
A particularly preferred first embodiment of the invention is disclosed in claim 1. This variant of the operating method according to the invention can be applied only to such discharge lamps having at least one dielectrically impeded electrode which have a starting aid. These are, in particular, aperture lamps whose discharge vessel is provided in part with a metallic coating, or whose discharge vessel is surrounded in part by an electrically conductive baffle, or discharge lamps whose discharge vessel bears a transparent, electrically conductive coating, for example an indium tin oxide layer (ITO layer). The abovenamed electrically conductive coatings or baffles are usually at frame potential and not only improve the electromagnetic compatibility of the lamps, but also act as capacitive starting aid.
In accordance with the first embodiment of the operating method according to the invention, for the purpose of igniting a dielectrically impeded discharge in the discharge lamp periodically recurring voltage pulses are applied to the at least one dielectrically impeded electrode, the voltage values of the voltage pulses being below the nominal operating voltage and being so low that the voltage pulses initially cause no ignition of the discharge. In the further course, the voltage values or voltage amplitudes of the periodically recurring voltage pulses are successively increased until a dielectrically impeded discharge forms in the discharge lamp. It has emerged that in the case of the abovementioned types of discharge lamps the ignition of a dielectrically impeded discharge already occurs with voltage pulses whose voltage values or voltage amplitudes are distinctly below the nominal operating voltage of the discharge lamp. The ignition of the dielectrically impeded discharge is performed in a gentle way, and no preferred discharge channels are formed. In particular, high voltage pulses are not applied to the electric components of the ballast immediately after connection. After performance of the ignition of a dielectrically impeded discharge in the discharge lamp, the voltage values or voltage amplitudes of the voltage pulses are advantageously successively further increased up to the nominal operating voltage of the discharge lamp.
In accordance with a second exemplary embodiment of the operating method according to the invention, a sequence of voltage pulses is applied to at least one dielectrically impeded electrode of the discharge lamp for the purpose of igniting a dielectrically impeded discharge, which pulses cause the discharge in the discharge lamp. Subsequently, the discharge is brought to extinction, and immediately after extinction of the discharge, there are applied to the at least one dielectrically impeded electrode periodically recurring voltage pulses which initiate a dielectrically impeded discharge in the discharge lamp. The operating method in accordance with the second embodiment can be used, in particular, with discharge lamps having at least one dielectrically impeded electrode which have no capacitive starting aid. The sequence of voltage pulses which generate the first discharge in the discharge lamp ensures the partial ionization of the discharge medium and generates free charge carriers. If preferred discharge channels should have formed in the discharge lamp during the first discharge caused by the sequence of voltage pulses, these channels collapse again after the end of the pulse sequence which had caused the first discharge. Immediately after the extinction of the first discharge, there are applied to the at least one dielectrically impeded electrode periodically recurring voltage pulses which have a sufficiently high voltage amplitude to cause a dielectrically impeded discharge. Since the dielectrically impeded discharge is initiated directly after the extinction of the first discharge, there are still sufficient free charge carriers present from the first discharge which permit the dielectrically impeded discharge to be ignited without a problem.